The present inventions relate to the field of fiber optic cables, and, more particularly, to fiber optic cables having optical fibers with optical performance characteristics being managed for signal transmission performance of high data rate systems.
Fiber optic cables are used to transmit telephone, television, and computer data information in indoor and outdoor environments in non-multiplexed and multiplexed optical transmission systems. In wave division multiplexed systems, optical performance characteristics play a significant role in maintaining high data rate transmission.
Optical attenuation, the loss in transmitted power, and chromatic dispersion, the differential transit time at adjacent wavelengths, are examples of optical performance characteristics in such transmission systems. Optical attenuation is typically due to absorption, scattering, and leakage of light from the optical waveguide and is customarily measured in a fiber, or cable, as a loss value in dB/km. Chromatic dispersion in fiber optic waveguides is the sum of material and waveguide dispersions and is generally measured as ps/(nmxc2x7Km). Differences in refractive index with respect to wavelength give rise to material dispersion. For silica-based glass used for optical fibers, material dispersion increases with wavelength through the commonly used communication range of about 0.9 xcexcm to 1.6 xcexcm. Material dispersion can have a negative or a positive sign depending on the wavelength.
Waveguide dispersion results from light traveling in both the core and cladding of an optical fiber. Waveguide dispersion is also a function of wavelength and the refractive index profile of the optical fiber. For example, a predetermined refractive index profile of the optical fiber can be selected to influence the wavelength dependency of wavelength dispersion therein, thereby influencing the chromatic dispersion at predetermined wavelength. Influencing wavelength and material dispersion affects combine to yield an overall positive or negative chromatic dispersion characteristic at any given point in a given optical fiber at a given wavelength. Optical performance concerns regarding pulse spreading caused by chromatic dispersion have created a need for dispersion compensating systems. Dispersion compensating systems employing, for example, positive and negative dispersion compensating fibers, are nevertheless subject to the optical performance constraints associated with optical attenuation.
A fiber optic cable design that acknowledges chromatic dispersion affects is described in U.S. Pat. No. 5,611,016. The patent pertains to a dispersion-balanced optical cable for reducing four-photon mixing in wave division multiplexing systems, the cable being designed to reduce cumulative dispersion to near zero. The dispersion-balanced optical cable requires positive and negative dispersion fibers in the same cable. Further, the positive dispersion aspect includes a dispersion characteristic defined as the average of the absolute magnitudes of the dispersions of the positive dispersion fibers exceeding 0.8 ps/(nmxc2x7km) at a source wavelength. In addition, the negative dispersion fiber characteristic requires the average of the absolute magnitudes of the dispersions of the negative dispersion fibers to exceed 0.8 ps/(nmxc2x7km) at the source wavelength. The aforementioned optical fibers are ribbonized, single-mode fibers designed for the transmission of optical signals in the 1550 nm wavelength region. The fibers are non-stranded or non-helically enclosed within a mono-tube cable, and are described as having an attenuation at 1550 nm of 0.22-0.25 dB/km, and attenuation at 1310 nm of  less than 0.50 dB/km. At defined parameters, the positive-dispersion characteristic is described as being +2.3 ps/(nmxc2x7km) and the negative-dispersion characteristic is described as being xe2x88x921.6 ps/(nmxc2x7km).
Other patents describe optical performance characteristics relating to a time division, rather than wave division, system. For example, U.S. Pat. No. 4,478,488 describes selective time compression and time delay of optical signals, without discussing the problems associated with attenuation or chromatic dispersion. A system is described using discrete channels having a dispersive section coupled to a standard multi-waveguide transmission section, and then another dispersive section. Signals are intended to propagate spatially out of phase, which can minimize channel coupling phenomena. An embodiment requires respective plastic coatings formed on twisted optical fibers, the coatings having varying diameters for varying the helix of the fibers in the cable. Individual fibers are spaced from the axis of the twist by different respective distances. This causes some fibers to twist more than others and extends the length of fiber located at the outside of the bundle, and the propagation times thereof, compared to one nearer the inside of the bundle. Using a multicore cable made up of cores embedded in a single cladding, each fiber is fixed at a helix that is different than the helix of any other fiber.
The present invention is directed to a fiber optic cable including a plurality of carriers respectively having at least one optical fiber therein, the carriers defining at least two layers generally disposed about a center area of the cable, the layers defining a relatively inner layer of carriers being closer to the center area, and an outer layer of carriers being relatively further from the center area, the inner and outer carrier layers each having a respective helix-plus-EFL value, a difference between the respective helix-plus-EFL values of the layers defining a differential range, the differential range being about 1% or less.
The present invention is also directed to a fiber optic cable system including first and second fiber optic cables, each of the first and second fiber optic cables having respective optical fibers disposed in carriers, the carriers defining at least two layers respectively in the fiber optic cables generally disposed about a center area of the respective fiber optic cables, the carrier layers defining a relatively inner layer of carriers being closer to the center area, and an outer layer of carriers being relatively further from the center area, the inner and outer carrier layers each having a respective helix-plus-EFL value, a difference between the respective helix-plus-EFL values of the layers in the fiber optic cables defining a differential range, the differential range being about 1% or less, and at least one optical fiber of the first fiber optic cable being optically connected to at least one optical fiber of the second fiber optic cable.
The present invention is further directed to a fiber optic cable system including first and second fiber optic cables, each of the first and second fiber optic cables having respective optical fibers disposed in carriers, the carriers generally arranged in at least one layer respectively in the cables and are generally disposed about a center area of the respective fiber optic cables, the first fiber optic cable having a different number of layers than the second fiber optic cable, the carrier layers of the fiber optic cables having a respective helix-plus-EFL value, a difference between the respective helix-plus-EFL values of the layers of at least one the fiber optic cables defining a differential range, the differential range being about 1% or less, and at least one optical fiber of the first fiber optic cable being optically connected to at least one optical fiber of the second fiber optic cable.
The present invention is still further directed to a method of manufacturing a fiber optic cable including selecting a predetermined helix and an excess fiber length (EFL) value for respective layers of carriers of the fiber optic cable, wherein the predetermined helix and EFL values are added together to calculate a respective helix-plus-EFL value for the respective layers of carriers, and maintaining the helix and EFL values for the respective layers of carriers so that a difference between respective helix-plus-EFL values of the respective layers is about 1% or less.